Delirium may be a complication of acute medical illness in up to 30% of hospitalized elderly. Few studies, however, have addressed its epidemiology, risk factors, or natural history with strict( criteria, uniform case ascertainment, and attention to potential confounders. We hypothesize that the occurrence of delirium identifies a group of frail elder with homeostatic impairment that might otherwise go clinically recognized. Such patients are likely to be at increase risk for long-term mortality, cognitive decline, and functional impairment. This study will examine the incidence, prevalence, and long-term prognosis of delirium among community-dwelling elderly admitted to general medical wards of university hospital. Behavioral indicators of delirium will be determined by screening all eligible admissions at 48 hour intervals enough their hospital stay. The presence of DSM-III-R criteria for delirium will be determined by a geriatrician and confirmed independently by a geropsychiatrist. The relation of delirium to potential risk factors such as age, sex, social support physical function, illness severity, and acute and chronic medical conditions will be examined. For the longitudinal phase of this study, two cohorts will be assembled - one of delirium cases, and a second consisting of controls matched for age, sex, and type of admission (emergency or elective)- Both groups will be followed at 6-month intervals for a total of 2 years to determine differences in long-term mortality, rates of institutionalization, and functional and cognitive decline. Survival analysis we examine the relationship of delirium to long- term outcomes, controlling for variables known to influence these outcomes. If delirium is an independent risk factor for loss of functional capacity in the elderly, this information will be useful directing interventions at frail elderly, and assist in planning clinical trials to prevent or ameliorate the consequences of delirium in hospitalized patients.